Electrocardiography is a non-invasive procedure used to sense from electrodes on the skin a signal representing electrical activity of the heart and to record and display the signal. A medical professional may then analyze the resulting electrocardiogram (EKG) to detect signs of heart abnormalities. Medical professionals are trained to determine whether an EKG is normal or abnormal and through the study of many EKGs both real and simulated, they decide whether an abnormal EKG indicates disease or is simply unusual. Skilled professionals effectively predict an amazing variety of abnormalities inside the heart from analysis of data provided by non-invasive EKGs.
When an EKG is performed, it is common to place six electrodes across the patient's chest and one electrode is placed on each limb. Focusing on the limb electrodes, EKG circuitry designates each electrode as positive or negative to create three “leads,” each with a view of the heart and an angle of orientation. The angles of orientation for leads I, II, and III are 0°, 60°, and 120°, respectively. Three additional, augmented leads are also created with different views of the heart and angles of orientation. The angles of orientation for leads aVL, aVR, and aVF are −30°, −150°, and 90°, respectively. Each electrode measures electrical activity within the heart and each lead is considered to represent a vector with an angle of orientation A representing an average direction and a voltage representing an amplitude. FIG. 1 illustrates the convention used to define the angles A.
Occasionally, the electrodes placed on the legs will be reversed. However, it is believed that such reversal is not often detected from an EKG chart and therefore is assumed to have no discernable effect on the EKG.